A waste heat steam generator is a heat exchanger which recovers heat from a hot flow of gas. Waste heat steam generators are used inter alia in gas and steam turbine systems (CCGT systems) which are predominantly used for generating power. A modern CCGT system conventionally comprises one to four gas turbines and at least one steam turbine, wherein either each turbine drives one generator respectively (multi-shaft system) or one gas turbine, together with the steam turbine on a shared shaft, drives a single generator (single-shaft system). The hot exhaust gases from the gas turbine(s) are used in the waste heat steam generator to generate steam. The steam is then fed to the steam turbine. Approximately two thirds of the electrical power are typically allocated to the gas turbine and one third to the steam turbine.
It should be mentioned for the sake of completeness at this point that basically different substances can be used as the moving fluid for the waste heat steam generator and the steam turbine. Reference will be made below by way of example to the use of water or water vapor since this is by far the most common moving fluid.
Analogously to the various pressure stages of a steam turbine, the waste heat steam generator conventionally also comprises a plurality of pressure stages with, during normal operation, different thermodynamic states of the water-steam mixture contained in each case. In the feed water or steam circuit the moving fluid passes in the course of its flow path firstly through an economizer in which residual heat in the exhaust gas stream is used to pre-heat the moving fluid. What is known as an evaporator adjoins the economizer and can preferably be constructed as a forced flow evaporator and in particular as what is known as a BENSON evaporator. The moving fluid is then in the form of steam or a water-steam mixture at the evaporator outlet, wherein possible residual moisture is separated in a separator positioned at this location. The onwardly-conveyed steam is consequently heated further in a superheater. The overheated steam then flows into the high pressure part of the steam turbine, expands there and is fed to the subsequent pressure stage in the steam generator. There it is overheated again and then introduced into the next pressure stage in the steam turbine. Adjoining the steam turbine outlet is a condenser in which the expanded steam is condensed and fed as feed water to a reservoir. A feed water pump finally conveys the feed water from the reservoir into the economizer again. The feed water flow rate is controlled by a control valve located downstream of the feed water pump.
The feed water flow rate in the feed water circuit, and in particular in the evaporator, is controlled as a function of the operating state of the waste heat steam generator and, connected therewith, of the current steam generator power. In the case of changes in load the evaporator flow-through should be changed as synchronously as possible to the heat introduction into the heating areas of the evaporator because, otherwise, a difference in the specific enthalpy of the moving fluid at the outlet of the evaporator from a desired value cannot be reliably avoided. Such an undesired difference in the specific enthalpy makes control of the temperature of the live steam issuing from the steam generator difficult and leads, moreover, to high material stresses and therewith to a reduced life of the steam generator.
To keep such differences in the specific enthalpy from the desired value, and, resulting therefrom, undesirably high temperature variations, as low as possible in all operating states of the steam generator, i.e. in particular in transient states or in the case of changes in load as well, the feed water flow controller can be constructed in the manner of what is known as a predictive or anticipatory design. The required feed water flow rate desired values should be provided as a function of the current operating state or for the operating state expected next, in particular in the case of changes in load as well. A control system which is very expedient in this respect is described in the unexamined and first European publications EP 2 065 641 A2 and EP 2 194 320 A1 which can both be attributed to the Applicants. Explicit reference is made to the entire disclosure of these documents.
An optimally flexible mode of operation is required of modern power stations in addition to a high level of efficiency. This includes the option of compensating frequency disruptions in the electric grid in addition to short start-up times and high load-change speeds. To satisfy these requirements the power station must be capable of providing increased outputs of, by way of example, 5% and more within a few seconds.
This is usually achieved in previously conventional CCGT power stations by increasing the load of the gas turbine. Under certain circumstances, however, it may be possible, in particular in the upper load range, that the desired increase in power cannot be provided solely, or cannot be provided quickly enough, by the gas turbine. Solutions are in the meantime also being pursued in which the steam turbine can and should also make a contribution to frequency stability, and primarily in the first few seconds following a power requirement.
This can occur by way of example by opening partially throttled turbine valves in the steam turbine or what is known as a stage valve, whereby the steam pressure upstream of the steam turbine is reduced. Steam from the steam accumulator of the waste heat steam generator located upstream is consequently withdrawn and fed to the steam turbine. A power increase is attained in the CCGT power station within a few seconds by way of this measure.
This additional power can be released in a relatively short time, so the delayed power increase can be at least partially compensated by the gas turbine (limited by its construction- and operation-related maximum load-change speed). As a result of this measure the entire power station block makes an immediate leap in power and as a result of a subsequent power increase in the gas turbine can also lastingly maintain this power level or even exceed it provided the system was in the partial load range at the time of the additionally required power reserves.
Permanent throttling of the turbine valves to provide a reserve always leads to a loss in efficiency, however, so for economic operation the degree of throttling should be kept as low as is absolutely necessary. Furthermore, some waste heat steam generator designs, thus for example forced flow steam generators, sometimes have a significantly lower storage volume than for example natural circulation steam generators.
In the method described above the difference in the size of the reservoir has an effect on the behavior of the steam turbine of the CCGT power station in the case of changes in power.